We plan to continue our past investigations of the control of macromolecular syntheses in cellular function but intend to shift increasingly our attention from bacteria to metazoan nerve cells. The organism which we have chosen for this purpose is the medicinal leech, Hirudo medicinalis, and the general problem which we intend to study is the control of the formation of receptor proteins for neurotransmitter chemicals in the postsynaptic membrane. It is our working hypothesis that this process forms the molecular basis of the plasticity of neuronal circuits by means of which the nervous system learns from experience. Before attacking this problem directly, however, we are trying to identify the neurons and their connections responsible for particular behavioral acts likely to be subject to experiential modification. Once we know that cellular nature of original and modified circuitry, we will be ready for the attempt to characterize the physiological and chemical factors underlying the macromolecular basis of these plastic changes. One such behavioral act, whose neuronal basis we elucidated in the past year, is the release of mucus polysaccharide from the leech skin and whose study we will continue in the coming year.